The present invention relates generally to the application of nickel-titanium alloys to medical devices. More precisely, the present invention is directed to creating double-butted tubing made of nickel-titanium alloys for use in medical devices.
Several interventional treatment modalities are presently used for heart disease, including balloon and laser angioplasty, atherectomy, and by-pass surgery. In typical coronary balloon angioplasty procedures, a guiding catheter having a distal tip is percutaneously introduced through the femoral artery into the cardiovascular system of a patient using a conventional Seldinger technique and advanced within the cardiovascular system until the distal tip of the guiding catheter is seated at the ostium of the coronary arteries. A guide wire is positioned within an inner lumen of a dilatation catheter and then both are advanced through the guiding catheter to the distal end thereof.
The guide wire is first advanced out of the distal end of the guiding catheter into the patient""s coronary vasculature until the distal end of the guide wire crosses a lesion to be dilated, then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy over the previously introduced guide wire until the balloon of the dilatation catheter is properly positioned across the lesion.
Once in position across the lesion, the balloon is inflated to compress the plaque of the lesion against the inside of the artery wall and to otherwise expand the inner lumen of the artery. The balloon is then deflated so that blood flow can be resumed through the dilated artery and the dilatation catheter can be removed therefrom.
One problem that can occur during balloon angioplasty procedures is the formation of intimal flaps which can collapse and occlude the artery when the balloon is deflated at the end of the angioplasty procedure. Another problem characteristic of balloon angioplasty procedures is the large number of patients who are subject to restenosis in the treated artery. In the case of restenosis, the treated artery may again be subjected to balloon angioplasty or to other treatments such as by-pass surgery, if additional balloon angioplasty procedures are not warranted. However, in the event of a partial or total occlusion of a coronary artery by the collapse of a dissected arterial lining after the balloon is deflated, the patient may require immediate medical attention, particularly in the coronary arteries.
A focus of recent developmental work in the treatment of heart disease is an endoprosthetic device referred to as a stent. A stent is a generally cylindrically shaped intravascular device that is implanted in a diseased artery to hold it open. The device is thus used to maintain the patency of a blood vessel immediately after intravascular treatments, and further reduces the likelihood of restenosis. In some circumstances, a stent can be used as the primary treatment device where it is expanded to dilate a stenosis and then left in place. Further details of stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); U.S. Pat. No. 5,421,955 (Lau); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.
One method and system developed for delivering stents to desired locations within the patient""s body lumen involves crimping a stent about an expandable member, such as a balloon on the distal end of a catheter, advancing the catheter through the patient""s vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway.
A limitation of some prior art stents, especially those of the balloon expandable type, is that they are stiff and inflexible. Often, the expandable type stents are formed from stainless steel alloys and are constructed so that they are expanded beyond their elastic limit. Such stents are permanently deformed beyond their elastic limits in order to hold open a body lumen and to maintain the patency of the body lumen. By the same token, since the material is stressed beyond its elastic limit into the plastic region, the material becomes stiff and inflexible.
There are several commercially available stents that are widely used and generally implanted in the coronary arteries after a PTCA (Percutaneous Transluminal Coronary Angioplasty) procedure, described earlier. Stents are also implanted in vessels that are closer to the surface of the body, such as in the carotid arteries in the neck or in peripheral arteries and veins in the leg. Because these stents are implanted so close to the surface of the body, they are particularly vulnerable to impact forces that can partially or completely collapse the stent and thereby block fluid flow in the vessel. Under certain conditions, muscle contractions might cause the stent to partially or totally collapse. Since balloon expandable stents are plastically deformed, once collapsed or crushed, they remain so, permanently blocking the vessel. These expandable stents might therefore pose an undesirable condition to the patient.
Such important applications as mentioned above have prompted stent designers to use superelastic or shape memory alloys in their stent to exploit the materials"" properties. Typically, the superelastic or shape memory alloy of choice is nickel-titanium, also known as nitinol. A nitinol stent is self-expanding and is highly resilient. As a result, a nitinol stent is not commonly deformed plastically when deployed, and remains highly resilient inside the body lumen. Because of this resilience, the self-expanding nitinol stent can encounter a deforming impact yet return to its initial shape. The chance of a permanent collapse of the self-expanding nitinol stent due to an impact force is thus minimized. An example of such shape memory alloy stents is disclosed in, for example, European Patent Application Publication No. EP0873734A2, entitled xe2x80x9cShape Memory Alloy Stent,xe2x80x9d which is hereby incorporated by reference.
The evolution of superelastic and shape memory alloy stents progressed to use of ternary elements in combination with nickel-titanium alloys to obtain specific material properties. As a general proposition, there have been attempts at adding a ternary element to nickel-titanium alloys as disclosed in, for instance, U.S. Pat. No. 5,885,381 to Mitose et al., which is hereby incorporated by reference.
Nickel-titanium alloys are frequently chosen for use in self-expanding stents due to their highly elastic behavior. Near equi-atomic binary nickel-titanium alloys are known to exhibit xe2x80x9cpseudoelasticxe2x80x9d behavior when given certain cold working processes or cold working and heat treatment processes following hot working. Generally speaking, xe2x80x9cpseudoelasticityxe2x80x9d is the capacity of the nickel-titanium alloy to undergo large elastic strains on the order of 8 percent or more when stressed and to substantially fully recover all strain upon removal of the stress. Substantially full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.
Pseudoelasticity can be further divided into two subcategories: xe2x80x9clinearxe2x80x9d pseudoelasticity and xe2x80x9cnon-linearxe2x80x9d pseudoelasticity. xe2x80x9cNon-linearxe2x80x9d pseudoelasticity is sometimes used by those in the industry synonymously with xe2x80x9csuperelasticity.xe2x80x9d
Linear pseudoelasticity results from cold working only. Non-linear pseudoelasticity results from cold working and subsequent heat treatment. Non-linear pseudoelasticity, in its idealized state, exhibits a relatively flat loading plateau in which a large amount of recoverable strain is possible with very little increase in stress. This flat plateau can be seen in the stress-strain hysteresis curve of the alloy. Linear pseudoelasticity exhibits no such flat plateau. Non-linear pseudoelasticity is known to occur due to a reversible phase transformation from austenite to martensite, the latter more precisely called xe2x80x9cstress-induced martensitexe2x80x9d (SIM). Linear pseudoelasticity has no such phase transformation associated with it. Further discussions of linear pseudoelasticity can be found in, for example, T. W. Duerig, et al., xe2x80x9cLinear Superelasticity in Cold-Worked Nixe2x80x94Ti,xe2x80x9d Engineering Aspects of Shape Memory Alloys, pp. 414-19 (1990).
Binary nickel-titanium alloys have been used in the medical field. Many medical device related applications exploit the non-linear pseudoelastic capabilities of nitinol. Examples include: U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 (Jervis); and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746; 5,720,754; and 6,004,330 (Middleman et al.), whose contents are hereby incorporated by reference.
One specific problem with self-expanding stents is the tendency for the proximal and distal end rings to collapse. This collapse compromises the mean lumen diameter (MLD). The collapse therefore detrimentally affects the actual versus expected deployment diameter of the stent. Also, collapsed end rings means that the scaffolding capability of the stent at its ends are diminished. Hence, what has been needed and heretofore unavailable in the prior art is a tubular shaped medical device that exploits the benefits of superelastic materials and structurally compensates for the inherently weak ends of a tubular structure.
The present invention in a preferred embodiment is directed to a stent for use in a body lumen, comprising a body having a tubular shape with opposite ends wherein at least one of the ends is butted; and a strut pattern formed from the tubular shape wherein the strut pattern includes a plurality of cylindrical rings generally coaxially aligned and interconnected by connecting links. The present invention stent is preferably made from a tubular shape that is butted at one or both ends, and each of the butted ends optionally coincides with a ring of the finished stent.
The present invention stent has a given wall thickness and the butted ends, as the term suggests, have a greater wall thickness than the wall therebetween. The thicker ends may be fashioned from a variety of tubing manufacturing processes, such as machining, cold working or drawing to thin the wall in between the ends, etc.
The preferred embodiment stent is further made from a nickel-titanium alloy. The nickel-titanium alloy, in a preferred embodiment, has a phase transformation temperature above 37 degrees C. The nickel-titanium alloy may be optionally cold-worked, heat treated, and may optionally include a ternary element to obtain desired engineering properties.
The resulting preferred embodiment self-expanding stent has all of the beneficial attributes of a superelastic medical device, and further exhibits great hoop strength especially for the rings that coincide with the double-butted ends. The risk of collapse at the stent ends therefore is dramatically reduced.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.